Membrane flow from the cell surface will be investigated using Paramecium caudatum as a model cell system. Endocytosis from the cell surface will be followed with time using pulse-chase studies incorporating electron-microscopic tracers - horseradish peroxidase and cationic ferritin - into the endosomes and polystyrene latex beads into phagosomes. All sites of endocytic-exocytic membrane flow on the cell surface of P. caudatum can be identified and studied since an alveolar system underlies the plasma membrane thus providing a semirigid surface on this cell. We will follow digestive vacuole membrane during its formation, the endosomes which are removed during vacuole condensation, secondary lysosome formation at advanced stages of digestion and, finally, the endosomes retrieved from the old digestive vacuole following defecation. We will also investigate two other sources of endosomes at the cell surface, those from parasomal sacs which give rise to "thick-membraned" vesicles and the "thin-membraned" remnants or cisternae resulting from trichocyst discharge. Using combinations of markers, microscopic monitoring of defecation and changes in temperature we have outlined a series of studies which allow us to separate and to follow the fate of each endosome population. Thus we will be able to detect the extent of endosome membrane intermixing in the cytoplasmic compartments. We believe that we will also be able to prove that direct membrane recycling, bypassing the Golgi apparatus and lysosomes, is operating in the return of digestive vacuole membrane from the cytoproct to the cytopharynx. Basic information on how normal healthy cells reutilize membrane for their best economic advantage will be obtained from this study.